They can live for up to 150 years, attaining lengths of more than 15m, and are believed to reach sexual maturity around the age of 30.

The fish are placid filter-feeders

Under the IUCN Red List of threatened species, they are categorised as "vulnerable" to extinction.

"Whale sharks, like many other shark species, are highly vulnerable to over-exploitation due to their long lifespan and low reproductive rate," commented Callum Roberts, of York University in the UK, who has researched whale sharks extensively in the Caribbean.

"They have been added to CITES' list of species threatened by international trade," he told the BBC News website, "but this will not protect them if they are caught by, for example, Taiwanese vessels and then consumed in Taiwan.

"So whale sharks are at risk, and the decline in size might be due to capture of large sharks."

There are also indications that the number of sharks visiting Australian waters may be decreasing, which would be additional evidence for a decline prompted by over-fishing.

Playing tag

Aims researchers are running a tagging programme in an attempt to plot whale shark migration routes between Australia, Asia and the eastern coast of Africa.

Specimens tagged in Australia have swum to Asian waters; last month a tag transmitted for days from the same location in Indonesia, apparently on land, leading researchers to suspect that the shark had been caught and the tag removed.

Either the meat is eaten, or the giant fins used as advertising boards for restaurants serving shark fin soup; livers are used for oil, and cartilage in traditional Chinese medicine.

Finding migration routes could help pinpoint areas where they are being caught.

"Many of the people doing the fishing are just local villagers with no other option," said Mark Meekan.

"If we know who they are, we can give them another option, and that option is very lucrative; the ecotourism industry in Ningaloo generates AU$70m (£28m; US$50m) a year, enough to support an entire town."

Longer term objectives of the Aims programme include finding out more about the life cycle of the whale shark.

The biggest mystery concerns breeding and reproduction; males and females live in largely segregated communities, but must come together somewhere to breed.

They are believed to bear live young, but sightings of pups are extremely rare.